InDesign CS or 2.0 will not open Quark 6.0 or Quark 5.0 documents. InDesign CS will only open Quark 4.x versions. I guess this is Adobe's way of getting you to buy CS and quarks way of trying to get you to not buy it.
Plugs ins for this might be available in the future but as of right now there are none. If you really need to get that quark document open in Indesign, you need to save your Quark 6.0 doc down to Quark 5.0 then down again to Quark 4.